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Widespread Twilight Zone Detected Around Clouds

 
killdamon
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05/07/2007 07:37 AM
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Widespread Twilight Zone Detected Around Clouds
by Staff Writers
Greenbelt MD (SPX) May 05, 2007
There seems to be something new under the sun -- in the sky, specifically -- that could complicate scientists' efforts to get a fix on how much the world will warm in the future. Greenhouse gases are not the only things in the air that influence the temperature of our atmosphere. Clouds and small airborne particles called aerosols also play an important and complicated role.

Now a new ingredient has been discovered: an extensive and previously unseen "twilight zone" of particles that represents a gradual transition from cloud droplets to dry particles.

In a study published last month, scientists from the Weizmann Institute, Rehovot, Israel, and NASA's Goddard Space Flight Center, Greenbelt, Md., document for the first time that air around clouds that was previously considered clear is actually filled with particles that are neither cloud droplets nor typical dry aerosols such as dust and air pollution. Worldwide, up to 60 percent of the atmosphere labeled as cloud-free in satellite observations is actually filled with this twilight zone of in-between particles, according to the study.

"With the highly sensitive Earth-observing instruments NASA has used since 2000, we can distinguish aerosols and clouds in greater detail than ever before," said Goddard's Lorraine Remer, a co-author on the study. "But the area around clouds has given us trouble. The instruments detected something there, but it didn't match our understanding of what a cloud or an aerosol looked like. What we think we're seeing is a transitional zone where clouds are beginning to form or are dying away, and where humidity causes dry particles to absorb water and get bigger."

Precisely accounting for everything in the atmosphere that can influence changes in global temperatures is critical to scientists' quest to accurately predict what Earth's climate will be in the future. The latest report from the Intergovernmental Panel on Climate Change, which assessed the potential risks of human-induced climate change, notes that the overall effect of clouds and aerosols on the amount of heat held in the atmosphere is still uncertain. Finding a previously unknown ingredient in the mix further complicates an already complex picture, but it also holds out the promise of resolving some nagging problems in climate change science.

"The effects of this zone are not included in most computer models that estimate the impact of aerosols on climate," said lead author Ilan Koren of the Weizmann Institute "This could be one of the reasons why current measurements of this effect don't match our model estimates." The study was published April 18 in the American Geophysical Union's Geophysical Research Letters. Atmospheric scientists have been aware of an indistinct "halo" of particles immediately surrounding individual clouds, which are sometimes visible to the naked eye. These are thought to be aerosols accumulating moisture and growing in size, or a cloud droplet shrinking as it evaporates. But the newly detected twilight zone extends far beyond single clouds to fill an entire cloud field.

The research team first came across evidence for this transitional zone in satellite measurements of aerosols that looked "suspicious," according to Remer. "After working with several years of data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra and Aqua spacecraft, we consistently saw what appeared to be elevated levels of aerosols near clouds. But we couldn't be sure that the instrument wasn't actually detecting stray light bouncing off of the clouds."

The region around clouds is difficult to accurately observe with instruments like MODIS because they operate like our eyes, collecting light reflected by objects below. Scientists interpret the different characteristics of the light received, matching them to known light patterns from different objects such as clouds. But clouds are notorious light scatterers, and the "glare" from the edge of clouds makes it hard to clearly detect what is around them. To be on the safe side, scientists mapping worldwide aerosols with MODIS avoid a 1-kilometer border around clouds.

To find out whether the apparent aerosol detection around clouds in the satellite data was real, Koren and his colleagues, including the late Yoram Kaufman of NASA Goddard, turned to an independent observing system on the ground: the NASA-sponsored Aerosol Robotic Network. The automated instruments in this global network minimize scattered light effects as they track the sun and take readings of the amount and size of aerosols in a narrow column of atmosphere between the instrument and the sun. When the sun is blocked by a cloud, the instrument doesn't make one of its regularly scheduled readings, which provides an indirect measure of the presence of a cloud.

Combining thousands of observations from 15 sites around the world, the researchers found that the amount of aerosol systematically increased as clouds got closer, as did the size of the particles. This held true regardless of whether the site was in a relatively clean setting or one where aerosols from air pollution or biomass burning were common.

"We found that the region affected by this cloud field 'twilight zone' extends to tens of kilometers beyond the identified cloud edge," said Koren. "This suggests that 30 to 60 percent of the atmosphere previously labeled as 'cloud-free' is actually affected by cloud-aerosol processes that reflect solar energy back into space."

Introducing this new factor could lead climate scientists to recalculate their best estimates of how Earth's atmosphere holds and reflects solar energy -- the key to accurately predicting the future of global warming. "Current estimates of the effect of aerosols on global temperatures, which is primarily cooling, may be too small because the large contribution from this transition zone has been overlooked," Remer said. "If aerosols are offsetting warming more than we thought, it's possible that warming could increase more than expected in the future if aerosols continue to decline, as has been reported recently."

This summer the scientists hope to get a closer look at the "twilight zone" and the hard-to-detect particles inside it with new measurements by the Aerosol Robotic Network and NASA aircraft. [link to www.terradaily.com]
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05/07/2007 07:56 AM
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Re: Widespread Twilight Zone Detected Around Clouds
I wonder how all those aerosols got up there.....

Actually, I don't wonder.
NOx

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05/07/2007 08:18 AM
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Re: Widespread Twilight Zone Detected Around Clouds
I wonder how all those aerosols got up there.....

Actually, I don't wonder.
 Quoting: Anonymous Coward 232152


Pray AC. Do tell :)
Anonymous Coward
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05/07/2007 03:05 PM
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Re: Widespread Twilight Zone Detected Around Clouds
Hmmm, makes you wonder about those noctilucent clouds that were appearing all over europe last year.
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05/07/2007 06:44 PM
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Re: Widespread Twilight Zone Detected Around Clouds
bump
killdamon (OP)

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05/07/2007 07:03 PM
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Re: Widespread Twilight Zone Detected Around Clouds
aerosols, define it. Where do they come from ? The only aerosols are those sprayed by planes. They are not from paint spray cans or hairspray cans and the like, cfc's were done away with many moons ago.
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Re: Widespread Twilight Zone Detected Around Clouds
by Mary Hardin and Ralph Kahn

Aerosols are tiny particles suspended in the air. Some occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. Averaged over the globe, aerosols made by human activities currently account for about 10 percent of the total amount of aerosols in our atmosphere. Most of that 10 percent is concentrated in the Northern Hemisphere, especially downwind of industrial sites, slash-and-burn agricultural regions, and overgrazed grasslands.

Scientists have much to learn about the way aerosols affect regional and global climate. We have yet to accurately quantify the relative impacts on climate of natural aerosols and those of human origin. Moreover, we do not know in what regions of the planet the amount of atmospheric aerosol is increasing, is diminishing, and is remaining roughly constant. Overall, we are even unsure whether aerosols are warming or cooling our planet.

Aerosols & Climate Change

Why do we care about aerosols?
Scientifc studies of aerosols

Related Articles:

Changing Our Weather One Smokestack at a Time
The effect of pollution on clouds and rainfall.

Every Cloud has a Filthy Lining
How the aerosols created by ship exhaust changes the structure of clouds.

Figure 1. Aerosol particles larger than about 1 micrometer in size are produced by windblown dust and sea salt from sea spray and bursting bubbles. Aerosols smaller than 1 micrometer are mostly formed by condensation processes such as conversion of sulfur dioxide (SO2) gas (released from volcanic eruptions) to sulfate particles and by formation of soot and smoke during burning processes. After formation, the aerosols are mixed and transported by atmospheric motions and are primarily removed by cloud and precipitation processes.

Why do we care about aerosols?
Aerosols tend to cause cooling of the Earth's surface immediately below them. Because most aerosols reflect sunlight back into space, they have a "direct" cooling effect by reducing the amount of solar radiation that reaches the surface. The magnitude of this cooling effect depends on the size and composition of the aerosol particles, as well as the reflective properties of the underlying surface. It is thought that aerosol cooling may partially offset expected global warming that is attributed to increases in the amount of carbon dioxide from human activity.

Figure 2a. (left) Clouds with low aerosol concentration and a few large droplets do not scatter light well, and allow much of the Sun's light to pass through and reach the surface.
Figure 2b. (right) The high aerosol concentrations in these clouds provide the nucleation points necessary for the formation of many small liquid water droplets. Up to 90% of visible radiation (light) is reflected back to space by such clouds without reaching Earth's surface.

Aerosols are also believed to have an "indirect" effect on climate by changing properties of clouds. Indeed, if there were no aerosols in the atmosphere, there would be no clouds. It is very difficult to form clouds without small aerosol particles acting as "seeds" to start the formation of cloud droplets. As aerosol concentration increases within a cloud, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere and decrease the amount of rainfall. In this way, changing aerosols in the atmosphere can change the frequency of cloud occurence, cloud thickness, and rainfall amounts.

If there are more aerosols, scientists expect more cloud drops to form. Since the total amount of condensed water in the cloud is not expected to change much, the average drop must become smaller. This has two consequences -- clouds with smaller drops reflect more sunlight (as explained in Figure 2a & b), and such clouds last longer, because it takes more time for small drops to coalesce into drops that are large enough to fall to the ground. Both effects increase the amount of sunlight that is reflected to space without reaching the surface. [link to earthobservatory.nasa.gov]
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Re: Widespread Twilight Zone Detected Around Clouds
twilizn
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Re: Widespread Twilight Zone Detected Around Clouds
Aerosols (aero/air solids or particulates/fine particles) come from volcanic eruptions, winds including tornadoes and cyclones, areal pollution (industry of all sorts, our chimneys and motor vehicles), aircraft and many other things. This thread is a delibrate attempt to promote balamoot or the socalled "chemtrails". Doofus maximus... erm killdamon... grow up will you :)

:danger_gas:

LONESTAR
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05/07/2007 08:02 PM
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Re: Widespread Twilight Zone Detected Around Clouds
This thread is a delibrate attempt to promote balamoot or the socalled "chemtrails".
 Quoting: NOx



Yes, you are partially correct. Chem/con trail formation is based upon aerosol particles acting as nucleated centers causing clouds and chem/con trails to form. It is not just the addition of water vapors to the atmosphere that causes trails. The word contrail, ie. condensation trail is itself a "con".
killdamon (OP)

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05/07/2007 08:09 PM
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Re: Widespread Twilight Zone Detected Around Clouds
This thread is a delibrate attempt to promote balamoot or the socalled "chemtrails".



Yes, you are partially correct. Chem/con trail formation is based upon aerosol particles acting as nucleated centers causing clouds and chem/con trails to form. It is not just the addition of water vapors to the atmosphere that causes trails. The word contrail, ie. condensation trail is itself a "con".
 Quoting: Anonymous Coward 233521

thumbs
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Re: Widespread Twilight Zone Detected Around Clouds
Yes, you are partially correct. Chem/con trail formation is based upon aerosol particles acting as nucleated centers causing clouds and chem/con trails to form. It is not just the addition of water vapors to the atmosphere that causes trails. The word contrail, ie. condensation trail is itself a "con".
 Quoting: Anonymous Coward 233521


Hm. Am I?

1rof1

Which part?

So are you and you are also bias :)

Ok. Here goes. Contrails/condensation trails are artificial cirriform clouds made by the exhaust of aircraft engines or wingtip vortices which precipitate a stream of tiny ice crystals in moist, freezing upper tropospheric air. They are sometimes beautiful to behold :)

An airplane's engine exhaust increases the amount of moisture in the air, which can push the water content of the upper tropospheric air past the saturation point. This causes condensation to occur, and the contrail to form. In the upper troposhere this water vapour emerges into a usualy cold environment and the local increase in water vapour density condenses into tiny water droplets and/or desublimates into ice. At high altitudes, supercooled water vapour requires a trigger to encourage desublimation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapor to rapidly turn to ice crystals. The aerosols/particulates that come from aircraft engine exausts are soot particles which are in fact the ones that act as the trigger for this desublimation which is logical is it not?

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